"Normal" microflora can cause serious infections in cancer patients and control of infections by prevention or elimination is essential. This study objective is to evaluate the in vitro effectiveness of new antibiotics against these bacteria and to monitor the susceptibility trends to antibiotics already in use. MK0787, an analogue of thienamycin, was a highly potent inhibitor of enterobactera, P. aeruginosa and gram-positive cocci, including those strains resistant to six lactam antibiotics; i.e., piperacillin, ticarcillin, carbenicillin, mezlocillin, azlocillin, and BAY K4999. Incidence of mezlocillin-resistant strains decreased to pre-use levels 1 year after mezlocillin use discontinuation. Piperacillin patterns remained the same. Carbenicillin resistance has increased among strains of P. aeruginosa and E. coli, as well as ticarcillin resistance among the latter strains and Klebsiella spp. The Moxalactam-amikacin combination exerted a synergistic or partially synergistic effect on about half of enterobacterial strains tested and about 90% of P. aeruginosa strains. Amikacin- piperacillin was also synergistic or partially so for most of the enterobacteria as well as P. aeruginosa. Moxalactam-piperacillin was usually synergistic for P. aeruginosa but not necessarily so for the enterobacteria.